As logic circuits continue to decrease in size and increase in speed, currently utilized semiconductor devices, such as transistors, may prove limiting. As such, novel types of semiconductor devices are under development to meet the requirements of high-speed circuitry. One such device is the resonant tunneling diode (RTD). Very generally, in devices of this type, the semiconductor layer structure includes thin layers of quantum wells that can permit low resistance, high speed electron tunneling. Such devices potentially provide increased high-speed switching, increased device density, and reduced power dissipation in logic circuits.
Currently, research on RTDs focuses on utilizing new fabrication technologies to reduce the size of these devices to the sub-micron level. Decreasing the size of these devices may reduce the capacitance and correspondingly increases the maximum frequency of oscillation for the device and, further, may reduce the requisite peak current for the device.
Decreasing the size of an RTD, however, has been limited by the increase in leakage current with the increase in surface-to-area ratio. Increased leakage current becomes a dominant factor in the current-voltage dependence of the device, and the increased leakage results in an increased valley current relative to the peak current. Thus, the peak-to-valley current ratio is reduced.
Nomoto, et al. reported successful fabrication of RTDs down to 20 nm in diameter. However, as noted above, the peak-to-valley ratios for these devices were limited by their dimensions. The peak-to-valley current ratio for the 20 mn device was less than 1.1 and thus the device is not practically useful in a circuit application. The 80 nm diameter device fabricated by the same researchers exhibited a somewhat better peak-to-valley current ratio of 1.2, with an attractively low peak current of 50 nA, but also exhibited a limiting peak current density of 103 A/cm2.
Smith, et al. reported fabricating a sub-micron RTD with an improved peak current density of 1.4×105 A/cm2 and a peak-to-valley current ratio of 2.0. However, while this device was 100 nm across, it was also 10 mm long, and thus had a net area of 1 mm2. This larger device size, while facilitating an improved peak-to-valley current ratio, also leads to higher power loss and higher peak current.
Therefore, it is desirable to have a sub-micron resonant tunneling diode with a favorable peak-to-valley ratio, lower peak current requirements and lower capacitance than larger RTDs.